The invention relates generally to the oxidation of ethylene to ethylene oxide over a supported silver catalyst. Such catalyst and processes are well known in the prior art. More specifically, the invention relates to an improved catalyst employing a characteristic support. The catalyst is activated in a manner intended to provide the optimum selectivity to ethylene oxide. Catalysts prepared according to the invention are further improved by post-depositing alkali metals in minor amounts on the activated silver catalyst.
Many patents show the use of supported silver catalysts for the oxidation of ethylene to ethylene oxide. For many years, promoting metals have been added to further improve performance, and in particular the alkali metals. The art in this field has been very extensive as may be seen in a lengthy review of these patents given in G.B. 2,043,481A. Such disclosures have been somewhat inconsistent in their teachings, as can be seen by comparing U.S. Pat. No. 2,238,474 in which sodium and lithium hydroxides were suggested as promoters and potassium and cesium were shown to act as poisons with U.S. Pat. No. 2,671,764 where rubidium and cesium sulfates were suggested as promoting compounds.
Although alkali metals were suggested generally in the earlier disclosures, more recent workers in the field have considered potassium, rubidium, and cesium as the preferred alkali metals. For example, see the series of patents to Nielson, et al. in which these materials were used in small amounts co-deposited with the silver, i.e. U.S. Pat. Nos. 3,962,136, 4,010,115, and 4,012,425. Still more recently the art has emphasized obtaining synergistic combinations of the alkali metals. For example see G.B. 2,043,481A cited above, and U.S. Pat. Nos. 4,212,772 or 4,226,782. In addition to their use in the preparation of fresh catalysts, the alkali metals have been used to rejuvenate used catalysts, as shown in U.S. Pat. No. 4,033,903 and a group of patents assigned to Hoechst, A.G. including U.S. Pat. Nos. 4,123,385 4,177,169 and 4,186,106. The art teaches that the alkali metals may be deposited either before the silver is placed on the support (pre-deposited), at the same time the silver is deposited (co-deposited), or subsequent to deposition of the silver (post-deposited). Examples of these techniques are given in U.S. 4,207,210 (pre-deposited), and the group of Nielson, et al. patents mentioned previously (co-deposited), and in U.S. Pat. Nos. 4,066,575 and 4,248,740, and G.B. 2,045,636A (post-deposited).
The useful amount of alkali metal was suggested to be quite wide in the older art. It was often indicated that large quantities, e.g. up to several percent of an alkali metal could be used. More recently, the art generally has taught that small quantities of alkali metals produce the optimum effect no matter when the silver and the alkali metals were deposited, although Kilty in U.S. Pat. No. 4,207,210 related the optimum amount to the surface area of the support. The art generally teaches that the optimum will be found in relatively low quantities, typically about 50 to 500 ppm by weight.
While the art generally teaches that alkali metals can be post-deposited (i.e. after the silver particles have been activated), it will be seen upon close study that either special methods are required to prepare a silver catalyst which can be promoted by alkali metals, or the catalysts must be deactivated through use or by artificial aging ("stabilization"). Otherwise, the art teaches that freshly made catalysts are not susceptable to promotion by alkali metals or rapidly lose what promotional effect is achieved. See for example, U.S. Pat. No. 4,033,903 which states that freshly prepared catalysts should be "stabilized" (that is, reduced in activity) through use or by heat treating, after which a much larger improvement in selectively can be achieved by deposition of potassium, rubidium, or cesium. In Example II it is shown that a catalyst activated at 200.degree. C. for 18 hours has essentially no response to the post-deposition of cesium. In U.S. Pat. No. 4,278,562 the same point is made and illustrated in comparative example 1. Example VII of British Pat. No. 1,413,251 teaches that post-deposition of potassium on a freshly prepared silver catalyst was inferior to the coincidental deposition of potassium with the silver. See also the patents issued to Hoechst A.G., which relate to the reactivation of used silver catalysts such as U.S. Pat Nos. 4,123,385, 4,186,106, and 4,177,169.
Patents which teach the successful promotion of freshly prepared silver catalysts by post-deposition of alkali metals include U.S. Pat. No. 4,066,575 where the silver is activated by heating in an inert atmosphere prior to deposition of an alkali metal. Example VII shows that activation of the silver in air provides a catalyst which shows little or no response to the post-deposition of cesium, while activation of the silver in nitrogen produces a catalyst which is significantly promoted by cesium. In U.S. Pat. No. 4,248,740 promotion by post-deposition of an alkali metal was achieved by heating only to temperatures of 50.degree. to 200.degree. C., followed by a washing with water or alcohol to provide a catalyst which can be promoted. Activation at temperatures above 200.degree. C. in air was shown in comparative Examples 2 and 3 to be clearly inferior. Published British patent application GB 2,045,636A also shows that activation at low temperatures produces a catalyst which can be promoted by post-deposition of an alkali metal, while Example 9 teaches that higher temperature activation in air produces a catalyst which is not promotable by post-deposition.
It has now been discovered that, contrary to the teachings of the prior art, that a silver catalyst can be successfully post-deposited with an alkali metal to provide improved selectivity, which is not merely a transient improvement, but is retained for a long period of operation. This result is achieved by proper selection of the catalyst support, activating the silver under controlled conditions, and post-depositing a small amount of an alkali metal in the manner disclosed hereinafter.